1. Field of the Invention
The present invention relates to an electrooptical device comprising thin film transistors formed over an insulating substrate and an IC chip formed over the same substrate, particularly to a structure of an active matrix type liquid crystal display device.
2. Description of the Related Art
In recent years, a technique for forming a thin film transistor (hereinafter referred to as a TFT) of a semiconductor thin film formed over a glass substrate has been developed. Moreover, an electrooptical device in which a voltage applied to an optical modulating layer held between a pair of substrates is controlled by a TFT to carry out an ON/OFF operation of light, has been developed.
Particularly, as a display such as a viewfinder of a video camera or a monitor screen of a note-sized personal computer, the demand for a liquid crystal panel using a liquid crystal as an optical modulating layer is rapidly increasing.
At present, the main current of development is a liquid crystal panel constituted by polysilicon TFTs using crystalline silicon films (typically polysilicon films) as semiconductor thin films. Since the operating speed of the polysilicon TFT is faster than an amorphous silicon TFT, it is possible to form a monolithic liquid crystal panel in which a pixel matrix circuit and a driving circuit (shift register and the like) are formed on the same substrate.
Further, it is desired to realize a system-on-panel in which not only a driving circuit such as a shift register but also a logic circuit such as a clock control circuit, a memory circuit, and a signal conversion circuit is formed on the same substrate.
Since such a logic circuit requires an operating speed as high as the GHz order, a polysilicon TFT is also required to have an extremely high operating speed. In order to realize such requirement, it is necessary to make a circuit element minute according to the scaling law.
However, it is very difficult to form a fine pattern with a wiring width of 1 μm or less on a generally used large glass substrate. For example, in the glass substrate, there occurs a problem such as undulation and shrinkage of the surface of the substrate. Moreover, it is very difficult to realize such an optical system as is capable of forming a fine pattern in a wide range, so that the formation of a fine pattern is restricted also by the development of a light exposure technique in an aspect.
Thus, at present, it is the limit to form a driving circuit such as a shift register on the same substrate (nevertheless, the operating speed is insufficient so that divided driving is carried out), and other logic circuit is provided by an external IC.